What happens to fat when we lose weight?
Interesting thought isn’t it. I thought thats easy, when we lose weight the fat is converted to energy, usually heat energy and lost that way. That would all seem to make sense and fits nicely with the idea of exercising to burn the calories off, job done. That’s until we realise this violates the law of conservation of mass.
So where does it actually go?
Fat is our bodies way of storing excess protein and carbohydrate and it’s stored in the adipocytes or fat cells. We store fat as triglyceride molecules. Losing weight means releasing or metabolising the fat from our bodies, but the chemicals that make up fat have to go somewhere due to the law of conservation of mass, they can’t just disappear into thin air as heat.
The average triglyceride in our fat cells has the chemical formula C55H104O6. This is important as it allows us to work out what happens to fat when we lose weight.
The breakdown of fat in our bodies is a complex process and involves many steps but the end result is as follows:
C55H104O6 + 78O2 --> 55CO2 + 52H2O + energy
Further tests show that when we lose weight 84% of the chemical mass of fat in our bodies is converted to carbon dioxide (CO2) and 16% is converted to water (H2O). Water in this sense is urine, poo, sweat, breath and any other bodily fluid.
So the answer is that we lose fat by converting it to carbon dioxide and water, the vast majority of carbon dioxide we breathe out through our lungs.
Eat less move more
As Meerman shows in his paper, the average 70 Kg human who in a 24 hour period rests for 8 hours, sleeps for 8 hours and does light activities for 8 hours that double the resting metabolic rate will exhale 740 g of CO2, equivalent to 202g of carbon.
Replacing one of the hours of rest with activity that is 7 times the resting metabolic rate, such as jogging, removes an additional 39g of carbon from the body or an extra 19% of carbon. A 100g muffin is about 20% of the bodies total daily energy requirement meaning that it only takes a small amount of excess food to wipe out your exercise gains. Never before has “eat less, move more” seemed so true.
Fat doesn't go away
Just as we cant lose the chemicals that make up our fat cells, as adults we cant reduce the number of fat cells we actually have. When we lose weight as an adult we keep the same number of fat cells, but they shrink with weight loss. It seems the number of fat cells we have in adulthood is set in adolescence. There is no clear evidence that having more fat cells results in faster weight gain. Even if it did, it doesn't change the fact that to lose weight we need to eat less and move more.
- Meerman R, Brown A. When somebody loses weight, where does the fat go? BMJ 2014;349:g7257
- Spalding KL et al. Dynamics of fat cell turnover in humans. Nature 2008;453:783-7